Oxygen is produced industrially in enormous quantities from air. Hitherto, substantially all industrially-produced oxygen has been separated from air by condensing the air and then fractionally distilling the liquid air to separate the oxygen from nitrogen and other gases. This liquification procedure consumes very large amounts of energy, since the boiling point of oxygen at atmospheric pressure is only 77.degree. K.
The industrial demand for oxygen is growing and is expected to increase greatly if and when large-scale production of synthetic fuels begins, since most of the processes for liquification of coal and similar methods of producing synthetic fuels require large quantities of oxygen. In view of the known disadvantages of the air liquification process, attention has recently been directed toward methods for the separation of oxygen from air at temperatures much closer to ambient. In principle, such separation methods are very simple; a solution is prepared containing a compound which can complex molecular oxygen in a manner similar to that of the known biological oxygen-containing proteins, myoglobin and hemoglobin, this solution is exposed to air or a similar oxygen-containing gas so that a large proportion of the oxygen-containing compound becomes complexed with oxygen, then the solution is removed from contact with the air and exposed to an environment in which the oxygen partial pressure is less than that in equilibrium with the oxygen-complexed compound, so that the oxygen-complexed compound gives up at least part of its oxygen, thereby releasing into the environment a gas much richer in oxygen than the air with which the solution was originally in contact (a small amount of nitrogen and other gases almost invariably comes over with the oxygen because of the solubility of the other gases in the solution). If further purification is desired, the process can be repeated to yield a gas even richer in oxygen.
Perhaps the most promising techniques for thus separating oxygen from air involve the use of so-called "immobilized liquid membranes". Such immobilized liquid membranes comprise a solid support, typically a synthetic polymer, which is inert to oxygen, together with liquid immobilized on the inert support. The support may have very fine holes therein so that the liquid cannot run through the porous material, a polymer film acting as the support may be swollen by contact with the liquid, or various other techniques may be used for immobilizing the liquid on the support. Air or some other oxygen-containing gas is passed over one side of the immobilized liquid membrane, while the gas which passes through the membrane is removed by pumping on the opposite side of the membrane. The oxygen "diffuses selectively" through the liquid membrane; in fact, since there is an oxygen partial pressure gradient between the two sides of the membrane, the oxygen molecules are carried in the form of a metal complex through the immobilized liquid membrane at a much greater rate than the rate at which other gases pass through the membrane. More detailed descriptions of such immobilized liquid membrane gas separation techniques are given in U.S. Pat. No. 3,396,510 issued August 13, 1968 to Ward et al; Chemical Engineering, July 13, 1981, page 63; Parrett Membranes Succeeding by Separating, Technology, March/April 1982, page 16; Scholander, Science, 131, 585 (1960), Bassett et al, Non-Equilibrium Facilitated Diffusion of Oxygen Through Membranes of Aqueous Cobaltodihistidine, Biochim. Biophys. Acta., 211, 194 (1970); Science News, March 6, 1982, page 151; and a Technical Brief, Oxygen Enrichment, published by Bend Research, Inc., 64550 Research Road, Bend, Oregon 97701 (Autumn 1981): the disclosures of all these documents are herein incorporated by reference.
Heretofore, synthetic oxygen carriers have proven unsuccessful in aqueous solutions. For instance, the many well-known 1:1 dioxygen adducts of cobalt(II) suffer from at least one of four major limitations when compared to the coboglobins: they exist only at low temperature, they do not exist in aqueous solutions, they rapidly react further to form the 2:1 .mu.-peroxo complexes, and they do not approach coboglobin in stability (as measured by the equilibrium constant Keq). A good understanding of this area can be gained by reading Jones et al, Synthetic Oxygen Carriers Related to Biological Systems, Chemical Reviews, 79(2), 139-179 (1979).
The present invention relates to salts of metal dry cave complexes useful for complexing molecular oxygen. For present purposes, a "dry cave" is a cavity in the vicinity of a vacant metal coordination site of the complex which can reversibly accommodate molecules. Such cavity can be made hydrophobic by providing interior walls consisting almost entirely of filled .pi.-orbitals. Thus, the metal dry cave complexes can reversibly bind small molecules such as dioxygen even in aqueous solutions. As such, the metal dry cave complexes can effectively emulate various protein complexes such as, for example, myoglobin, hemoglobin, and the like.
Heretofore, nickel dry cave complexes have been proposed (Schammel, Thesis, The Ohio State University, Columbus, Ohio, 1976; Busch et al, Control of Potentials of Metal Ion Couples in Complexes of Macrocyclic Ligands by Ligand Structural Modifications, Reprint of ACS Symposium Series, No. 38. Electrochemical Studies of Biological Systems, American Chemical Society, 1977). Cobalt and iron derivatives were attempted, but it is now known that such attempts were fruitless.